A seat belt device installed in a vehicle in which a seat belt (webbing) is wound and extracted using an electric actuator is known. In this kind of seat belt device, the extracting amount and winding amount of the seat belt are detected, and tension applied to the seat belt is adjusted so that the belt does not become loose or too tight while the vehicle is traveling. Usually, a rotation detection unit detects the rotation of the spindle of the retractor, thereby detecting the extracting amount and the winding amount of the seat belt. Thus, the rotation of the spindle is controlled in accordance with the extracting and winding amounts, as can be seen for example, in Patent document 1: Japanese Patent Unexamined Publication No. JP-A-2005-297781).
The seat belt device described in Patent document 1 has, as means for detecting the rotation of the spindle, a disc rotating in synchronization with a spindle and a permanent magnet and a magnetic sensor disposed so as to face opposite each other with the disc interposed therebetween. Transmitting sections for transmitting a magnetic field and shielding sections for shielding the magnetic field are provided alternately on the disc in the circumferential direction thereof, and the magnetic sensor detects the change in the magnetic field when the disc passes through between the permanent magnet and the magnetic field, thereby detecting the rotation of the spindle.
In the seat belt device described in Patent document 1, although the rotation of the spindle of the retractor is detected on the basis of the change in the magnetic field, if a disturbance magnetic field is applied to the rotation detection unit, there is a problem that the detection performance may be easily affected.
In consideration of the above-mentioned circumstances, an object of the present invention is to provide a seat belt device capable of being less affected by the disturbance magnetic field and capable of detecting the rotation of a spindle with high reliability.